This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-280666, filed Sep. 30, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a field emission device, and relates, more particularly, to a field emission device having a three-electrode structure of a cathode, an anode and a gate electrode.
There have been proposed various field emission type cold cathodes. Among others, a tip emitter called a Spindt type emitter and a surface conduction emitter are representative types. In recent years, a method using a carbon nanotube that is stable with a low work function has also been proposed.
FIG. 1 shows a cross section of a tip emitter. This emitter has a sharp front end of a tip emitter 170 formed on a cathode 120, with the front end having a curvature radius of a few nanometers to a few dozens of nanometers. The tip emitter emits cold electrons based on a strong electric field that is concentrated at the front end. In other words, an electric field is formed between the front end of the emitter 170 and a gate electrode 140 formed on a first insulation layer 130 on the cathode 120, and electrons are emitted from the front end of the tip emitter 170. Therefore, in order to emit electrons at a low voltage, it is ideal to set a distance between the gate electrode 140 and the emitter 170 as short as possible. The emitted electrons are drawn to a direction of an anode (not shown) disposed above the tip emitter 170. However, each electron has an initial speed in a horizontal direction at the time of the emission, and therefore, the electron beams are spread in a lateral direction.
In order to prevent this spread of the electron beams, a control electrode 160 is disposed above the gate electrode 140 as shown in FIG. 1. In this case, it is necessary that an aperture diameter of the gate electrode 140 and an aperture diameter of the control electrode 160 are set to have a suitable ratio. In order to install the control electrode 160, it is necessary to install an insulation layer 150 on the gate electrode 140 and then to install the control electrode 160 on the insulation layer 150. In order to implement this installation process, a high-precision aligner is necessary. Therefore, this has a drawback in that not only the installation process increases, but also the facility necessary for the manufacturing becomes expensive.
In the mean time, in the case of the surface conduction emitter, an electron emitter is provided on a conductive thin film that extends over a pair of electrodes (an emitter electrode and a gate electrode) that are formed on a substrate. When an electric field is applied to the electrodes on both ends of the electron emitter, electrons are drawn out in a horizontal direction from an emitter electrode, and force is applied to the gate electrode provided on the substrate. Thus, the electrons are emitted in a horizontal direction. An acceleration electrode is provided above the electron emitter, and a part of the emitted electrons fly to the acceleration electrode. However, this efficiency is low, and the electrons are emitted in a parabolic direction in stead of a vertical direction from the substrate. Therefore, the electrons that collide against the acceleration electrode are deviated from the normal line of the electron emitter. Because of this phenomenon, when the field emitter is applied to an image display unit, beams are dispersed. As a result, there occurs a leakage of beams to adjacent pixels, or a high-efficient light emission is not obtained.
FIG. 2 is a perspective view showing one example of a surface conduction emitter disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-250018. This surface conduction emitter solves the leakage of the beams to adjacent pixels by narrowing the emitted electron beams. In order to solve the above phenomenon, there are provided electrodes 122a and 122b that form an equipotential surface of approximately a U shape in a direction orthogonal with a direction of voltage application between a pair of electrodes 123a and 123b, on a surface that is defined by the direction of voltage application between the pair of electrodes 123a and 123b and a direction of an electric field application by an acceleration electrode (above the electrodes 123a and 123b not shown) that works on the emitted electrons.
However, according to the surface conduction emitter, in order to form the approximately U-shaped equipotential surface, it is necessary to set the electron emitter at the center of the device electrode, and it is also necessary to strictly adjust the device formation and the height of the wiring electrode.
In order to solve the difficulty of the above manufacturing methods, a four-electrode type field emitter has been proposed in Jpn. Pat. Appln. KOKAI Publication No. 8-293244. FIG. 3 shows the four-electrode type field emitter. The disclosed four-electrode structure consists of a cathode 131, a control electrode 134, a gate electrode 133, and an anode 136. According to this method, neither a tip emitter nor a surface conduction emitter is used, but a material of a low work function is used as an electron emission layer 135. A shape of electron beams is narrowed by the substrate (cathode) 131 on which the electron emission layer 135 has been formed, the beam-forming electrode (control electrode) 134 that has been formed on the electron emission layer 135 by surrounding the electron emission layer, and the gate electrode 133 that has been formed on an insulation layer 132 on the beam-forming electrode 134.
However, according to this emitter, it is unavoidable that the process also becomes complex as it is necessary to form the control electrode in a similar manner to that of the emitter shown in FIG. 1.
Further, Jpn. Pat. Appln. KOKAI Publication No. 9-82215 has disclosed an emitter that has a large number of field emission tips having fine sizes within the electron emission surface. Further, there has been proposed a structure that has a ratio of a distance between a gate and an emitter to an aperture diameter (short diameter) set to 1 to 2 or higher so that the large number of field emission tips can have an approximately equal opportunity of emitting electrons. Based on this structure, it has been intended to be able to drive approximately homogeneously an emitter made of a bundle of nanometer-sized wires. However, this disclosure has an object of driving approximately homogeneously the emitter made of a bundle of nanometer-sized wires. This disclosure does not intend to restrict the spreading of the orbit of electron emission. Thus, this disclosure describes that it is desirable to have a control electrode without particularly limiting the electrode structure.
As explained above, as it is difficult to control the direction of electrons emitted by the field emitter that has a three-electrode structure of a cathode, an anode and a gate electrode, it has been conventionally assumed that a four-electrode structure having a control electrode in addition to the three electrodes is necessary. However, the four-electrode structure has a complex structure around the electron emitter. Further, this structure involves a difficulty in the manufacturing aspect as the electron emitter must be installed at the center of the electric field.
It is an object of the present invention to provide a field emission device having a three-electrode structure that can be manufactured easily and that can control the direction of emitted electrons.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a field emission device consisting of three electrodes, the field emission device comprising:
an emissive material formed on a cathode on a substrate;
an insulation layer formed to surround the emissive material;
a gate electrode formed on the insulation layer and having an opening for passing electrons emitted from the emissive material; and
an anode opposing to the emissive material, wherein
L/Sxe2x89xa71, where S represents an aperture diameter of the opening, and L represents a typical shortest passing distance of the electrons emitted from the emissive material to the gate electrode.
According to a second aspect of the invention, there is provided a field-emission type display unit essentially consisting of three electrodes, the field-emission type display unit comprising:
a substrate;
a cathode layer formed on the substrate;
an insulation layer formed on the cathode layer, and having a plurality of first openings;
a gate electrode formed on the insulation layer, and having a plurality of second openings corresponding to the plurality of first openings, each of the second openings having the same aperture diameter as that of each of the first openings;
an electron emission layer formed on the cathode layer exposed through the first and the second openings;
a transparent plate disposed to face a surface of the substrate on which the cathode layer is formed, via a frame provided on a periphery of the substrate;
an anode layer formed on a surface of the transparent plate facing the cathode layer; and
a phosphor layer formed on the anode layer, wherein
L/Sxe2x89xa71, where S represents the aperture diameter of the plurality of first openings, and L represents a typical shortest passing distance of the electrons emitted from the emissive material to the gate electrode.
More specifically, the electron emission layer of the field emission device or the display unit of the present invention is formed at the bottom of a deep opening so that an electric field is applied to the emitted electrons in a direction approximately vertical to the electron emission layer. With this arrangement, only the electrons of which speed component is large in a direction approximately vertical to the electron emission layer pass through the opening of the gate electrode and reach the anode. Thus, it is possible to make narrow the orbit of the electrons that have passed through the opening of the gate electrode and proceed to the anode. Therefore, it is possible to control the orbit of the electrons in a three-electrode structure that does not have a control electrode. In a three-electrode structure having a simple structure, the relationship of 1 greater than L/Sxe2x89xa71/2 disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-82215 cannot sufficiently function to restrict the spreading of the orbit of the electron emission. The spreading can be restricted when the relationship is set to L/Sxe2x89xa71. This is a fact that has been made clear for the first time by the present inventor.
Further, it is preferable that an average surface density of the plurality of openings is set to 1 pc/xcexcm2 or above. According to Jpn. Pat. Appln. KOKAI Publication No. 9-82215, the homogeneity of electron emission points is improved by taking a large number of emission points within a single opening. However, based on this structure, it is difficult to decrease the variance among electron emitters having individual openings. According to the present invention, the electron emitters having individual openings are disposed closely to decrease the variance. In other words, the average surface density is set to 1 pc/xcexcm2 or above. With this arrangement, even if there is a variance among the volumes of electrons emitted from individual openings, the volumes of the emitted electrons can be homogenized on average. This has an effect of restricting the variance of luminance between pixels when the invention is applied to a display unit.
The opening relating to the present invention can take a circular shape, an elliptical, or a polygonal shape, and the shape is not particularly limited. The diameter of the opening is a diameter of a circle when the opening takes a circular shape (see FIG. 4A), and the diameter of the opening is a short diameter when the opening takes an elliptical (see FIG. 4B). The diameter of the opening is a diameter of an inscribed circle when the opening takes a triangular shape or a square shape (see FIGS. 4C and 4D). The diameter of the opening is a diameter of a circle that is inscribed to longer parallel sides when the opening takes a parallelogram (see FIG. 4E). In these FIGS. 4A to 4E, a reference number 6 denotes an opening.
In spite of the improved control of the spreading of electrons, a part of the electrons that pass through the opening have a speed component in a direction parallel with the electron emission layer. These electrons work to spread the orbit of electrons when they pass through the opening. However, when a relationship between a thickness of the gate electrode Lg and a typical shortest distance L is set to Lg/Lxe2x89xa70.75, it is possible to restrict the spreading of the orbit of the electrons to a negligible level while securing the volume of electrons proceeding to the anode electrode when the invention is applied to a display unit or the like.
More specifically, based on the setting of the relationship of L/Sxe2x89xa71, a majority of the electrons are emitted to a direction approximately vertical to the electron emission layer, and a part of electrons that have the speed component in a direction parallel with the electron emission layer are elastically scattered by the insulation layer. However, when the electron emission layer is formed at the bottom of the deep opening, the orbit of the emitted electrons in the vertical direction can be easily corrected. Further, even if electrons take a distance exceeding the shortest distance L, those electrons having the parallel component collide against the gate electrode that has a predetermined thickness, and are absorbed by the gate electrode. On the other hand, when the thickness of the gate electrode is too much, the volume of those electrons that are absorbed by the gate electrode when passing through the gate electrode increases, and it becomes impossible to secure a necessary current. Therefore, the brightness changes on the display of the display unit. In order to secure this necessary brightness, the relationship of Lg/Lxe2x89xa60.75 has been set.
Further, it is preferable that the emissive material is formed on a plane on the cathode layer, and is at least one selected from Pd, Cs, LaB6, graphite, carbon and diamond.
Further, it is preferable that a space formed by the substrate, the transparent plate and the frame is in vacuum.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.